12-year-old Curtis and his older brother Ricky are growing up in one of London's most volatile neighbourhoods. A minor street clash escalates into a cycle of tit for tat violence with devastating consequences for both of them, their family and friends.

Directed by the former documentarist Saul Dibb (Tottenham Ayatollah), Bullet Boy is a sober, sombre, generally unsensationalised portrait of two black brothers from Hackney, north-east London, one only just out of his teens, the other having just entered them, and both caught up in events over which they ultimately have little control.

Although he emerges from prison at the start of the film, and is still on probation, Ricky (Ashley Walters) is acutely conscious of the effect his lifestyle has had on his family in general and his brother in particular. When a minor traffic accident threatens to turn into full-blown confrontation, purely for the sake of macho bravado and saving face, Ricky is the one attempting to mediate, desperate not to get sucked back into a world he's trying to escape - but his instinctive loyalty to the ironically-named Wisdom (Leon Black) ends up having fatal consequences for both of them.

But Ricky is at least old enough to understand where his actions might lead: he's already found out the hard way. Twelve-year-old Curtis (Luke Fraser) has no such advantage, hero-worshipping both his brother and the trappings of a lifestyle that he seems to represent (though much of this is carefully constructed bravado). When Curtis and his friend Rio discover the gun that Ricky has been hiding (in order to keep it out of Wisdom's trigger-happy hands), they take it outdoors and essentially re-enact the shoot'em'up video games that they're addicted to, with inevitable but still horrifying results.

What most impresses about Bullet Boy is its quietly understated realism and refusal to opt for finger-wagging moralising - which in any case would have been redundant, since Ricky and Curtis are well-drawn enough for the film's message about the debilitating, pointlessly escalating effect of black-on-black violence to come through loud and clear.

Dibb, shooting in anamorphic widescreen (an unusual choice for an essentially claustrophobic film) makes particularly effective use of his Hackney locations, where run-down tower blocks overlook the marshes, and a freshly-killed dog (a crude revenge gesture by Wisdom) remains in the canal for days. In the face of such a hope-sapping environment, the boys' mother (Claire Perkins) can do little more than wring her hands and turn to religion, leaving them to fend helplessly for themselves. At least Curtis' final gesture, though small in scale and unlikely to achieve much in itself, strikes a faint note of optimism.